As next generation lighting devices or display devices, display devices using light-emitting elements (organic EL elements) in which organic compounds are used for light-emitting substances have been developed rapidly because of their advantages of thinness, lightweightness, high speed response to input signals, low power consumption, and the like.
In an organic EL element, voltage application between electrodes between which a light-emitting layer is provided causes recombination of electrons and holes injected from the electrodes, which brings a light-emitting substance into an excited state, and the return from the excited state to the ground state is accompanied by light emission. Since the wavelength of light emitted from a light-emitting substance is peculiar to the light-emitting substance, use of different types of organic compounds for light-emitting substances makes it possible to provide light-emitting elements which exhibit various wavelengths.
In the case of display devices which are expected to display images, such as displays, at least three-color light, i.e., red light, green light, and blue light are necessary for reproduction of full-color images. Further, in lighting devices, light having wavelength components evenly spreading in the visible light region is ideal for achieving a high color rendering property, but actually, light obtained by mixing two or more kinds of light having different wavelengths is often used for lighting application. Note that it is known that mixing light of three colors of red, green, and blue allows generation of white light having a high color rendering property.
Light emitted from a light-emitting substance is peculiar to the substance as described above. However, important performances as a light-emitting element, such as a lifetime, power consumption, and emission efficiency, are not only dependent on the light-emitting substance but also greatly dependent on layers other than the light-emitting layer, an element structure, properties of a light-emitting substance and a host material, compatibility between them, carrier balance, and the like. Therefore, there is no doubt that many kinds of light-emitting element materials are necessary for a growth in this field. For the above-described reasons, light-emitting element materials with a variety of molecular structures have been proposed (e.g., see Patent Document 1).
As is generally known, the generation ratio of a singlet excited state to a triplet excited state in a light-emitting element using electroluminescence is 1:3. Therefore, a light-emitting element in which a phosphorescent material capable of converting the triplet excited energy to light emission is used as a light-emitting material can theoretically realize higher emission efficiency than a light-emitting element in which a fluorescent material capable of converting the singlet excited energy to light emission is used as a light-emitting material.
As a host material in a host-guest type light-emitting layer or a substance contained in each carrier-transport layer in contact with a light-emitting layer, a substance having a wider band gap or a higher triplet excitation level (a larger energy difference between a triplet excited state and a singlet ground state) than a light-emitting substance is used for efficient conversion of excitation energy into light emission from the light-emitting substance.
However, most of substances used as host materials in the light-emitting elements are fluorescent, and the triplet excited state of the substance is at a lower energy level than the singlet excited state thereof. Therefore, a host material needs to have a wider band gap in the case where a phosphorescent material is used as a light-emitting material than in the case where a fluorescent material is used as a light-emitting material even when the phosphorescent material and the fluorescent material have the same emission wavelength.
Accordingly, to efficiently obtain phosphorescence having a shorter wavelength, a host material and a carrier-transport material each having an extremely wide band gap are necessary. However, it is difficult to develop a substance to be a light-emitting element material which has such a wide band gap while enabling a balance between important characteristics of a light-emitting element, such as low driving voltage and high emission efficiency.